1. Field of the Invention
The present invention relates to an exhaust-gas recombustion system which is arranged in the exhaust line of an internal combustion engine, and specifically to a system which can promote activation of a catalytic converter by recombusting exhaust gases at the aid of secondary air pumped to exhaust manifolds or cylinder-head exhaust ports.
2. Description of the Prior Art
Recently, there have been proposed and developed various exhaust-gas recombustion systems which utilize secondary air to promote chemical reactions that reduce exhaust-gas pollutants. The exhaust-gas recombustion system usually cooperates with a catalytic converter disposed in the exhaust line of the engine to promote a chemical reaction between a catalyst and the pollutants. As is generally known, a catalytic converter is provided in an exhaust system of the engine for converting harmful exhaust gases such as unburned hydrocarbons (HC), carbon monoxide (CO) to harmless gases such as harmless water vapor and carbon dioxide (CO.sub.2). However, during a cold start of the engine in cold weather, it is difficult to obtain a sufficient activation of the catalytic converter, since a temperature of the catalyst support arranged in the catalytic converter cannot rise quickly to a temperature enough to provide adequate catalytic action, owing to cold engine. During a cold start, an insufficient activation of the catalytic converter results in high exhaust emissions. Thus, it is advantageous quickly to warm the catalyst support in the catalytic converter to a desired temperature in order to assure a rapid activation of the catalytic converter. For this reason, an exhaust-gas recombustion system employing a secondary air injection device is generally provided in the exhaust system of the engine to burn any HC and CO in the exhaust gases coming out of the exhaust ports by additional oxygen in the secondary air. Such exhaust-gas recombustion systems using a secondary air injection device have been disclosed in Japanese Patent First Publication (Tokkai Heisei) No. 3-134241 and in Japanese Utility Model First Publication (Jikkai Showa) No. 59-141118.
FIG. 1 shows a conventional exhaust-gas recombustion system with a secondary air injection device as described in the above Japanese document No. 3-134241. In FIG. 1, a piston 4 is reciprocatingly disposed in a cylinder 1A defined in a cylinder block 1 of an internal combustion engine. A combustion chamber 5 is defined on the top of the piston 4. An exhaust valve 7 is arranged between the combustion chamber 5 and an exhaust port 3 and opened to allow burned gases to exhaust from the cylinder 1A to an exhaust manifold 6 during the exhaust stroke. The exhaust manifold 6 is fitted through a gasket 8 to an engine cylinder head 2 to interconnect the exhaust ports 3 and the exhaust pipe (not shown). As seen in FIG. 1, the exhaust port 3 is curvedly arranged between the combustion chamber 5 and the inlet port of the exhaust manifold 6, while the exhaust manifold 6 is curvedly arranged between the exhaust ports 3 and the exhaust pipe. In order to assure a smooth exhaust-gas flow, the axial line of the curved exhaust ports 3 and the axial line of the curved exhaust manifold 6 are continuously connected to each other, such that the outside curved wall 3a of the exhaust port 3 and the outside curved wall 6a of the exhaust manifold 6 are formed continuously and that the inside curved wall 3b of the exhaust port 3 and the inside curved wall 6b of the exhaust manifold 6 are formed continuously. As clearly seen in FIG. 1, a secondary air injection tube 9 is attached to the exhaust manifold 6 in such a manner as to penetrate the outside curved wall 6a of the exhaust manifold 6. The air-injection tube 9 is bended so that the air-injection nozzle 9A of the air-injection tube 9 is arranged perpendicularly to the axial line of the curved exhaust manifold 6 or the exhaust-gas flow. The air-injection tube 9 is projected from an air manifold 10 into which the secondary air is pumped and introduced. With the above arrangement, the previously noted prior art exhaust-gas recombustion system suffers from the drawback that a smooth introduction of the secondary air is disturbed owing to residual exhaust gases left in the exhaust ports 3 close to the exhaust valve 7. In this case, an insufficient amount of the secondary air comes into contact with high-temperature and high-pressure exhaust gases in the exhaust manifold 6 having a relatively low temperature as compared with the engine block. This causes an insufficient recombustion of the exhaust gases coming out of the combustion chamber 5. Specifically during a cold start of the engine, the exhaust-gas temperature cannot rise to a desired temperature enough for a rapid activation of the catalytic converter which is provided downstream of the exhaust manifold 6, owing to such insufficient recombustion. In addition to the above, the above-mentioned prior art exhaust-gas recombustion system as well as the catalytic converter cannot sufficiently recombust unburned HC and CO content in the exhaust gas, during a cold start.
Referring now to FIGS. 2 and 3, there is shown one example of a mounting arrangement of the air-injection tubes 9A, 9B, 9C and 9D on branch pipes 6A, 6B, 6C and 6D of the exhaust manifold 6 in cars equipped with a manifold type catalytic converter 14. One such manifold type catalytic converter has been disclosed in Japanese Utility Model First Publication (Jikkai Showa) No. 59-34012. The aforementioned air-injection tube as generally referred to is represented by reference numeral "9" as illustrated in FIG. 1. As seen in FIG. 3, the manifold type catalytic converter 14 is attached to one side of the cylinder head 2. The manifold type catalytic converter 14 comprises a plurality of flanges 15 provided for firmly mounting the catalytic converter assembly on the cylinder head 2, four branch pipes 6A, 6B, 6C and 6D, and a converter shell 17. The converter shell 17 is oval in cross-section and accommodates the catalyst support therein. In brief, the manifold type catalytic converter 14 is a compact exhaust-emission control device obtained by assembling the catalytic converter and the exhaust manifold as a unit. It is advantageous that the manifold type catalytic converter 14 is arranged in the vicinity of the exhaust ports upstream of the exhaust system and that the catalytic converter 14 is capable of treating high-temperature and high-pressure exhaust gases just coming out of the exhaust ports with a high catalytic conversion efficiency. The flanges 15 include through openings 15A each of which communicates with either one of the exhaust ports 3. As clearly seen in FIG. 1, one end 6E of each branch pipe 6A, 6B, 6C and 6D is connected into either one of the through openings 15A by welding. As seen in FIG. 2, the respective ends 6E of the branch pipes are arranged in parallel with each other. The other ends 6F of the branch pipes 6A, 6B, 6C and 6D are connected to a substantially dome-shaped upper end 17A of the catalytic converter. The secondary air manifold 10 is connected through a flange 10A to a compressed air source (not shown). The air manifold 10 is traditionally made of a cylindrical hollow pipe. The upstream ends of the air-injection tubes 9A, 9B, 9C and 9D are respectively connected to the air manifold 10, while the downstream ends of the air-injection tubes are respectively connected to the ends 6E of the branch pipes 6A, 6B, 6C and 6D such that the air-injection nozzle is projected into the corresponding pipe end 6E. As seen in FIG. 2, the respective air-injection tubes 9A, 9B, 9C and 9D are arranged in parallel with each other. In the previously noted construction of the air-injection device, the secondary air is distributed from the compressed-air source through the air manifold 10 to the respective air-injection tubes 9 and consequently to the respective branch pipes 6. The distributed secondary air promotes chemical reactions that reduce exhaust-gas pollutants by recombusting unburned HC and CO in the exhaust ports 3 and each of the branch pipes 6A, 6B, 6C and 6D. Thus, the manifold type catalytic converter 14 is rapidly activated owing to the quickly warmed catalyst support. However, in the above-mentioned air-injection tube arrangement, a secondary air distribution between the four air-injection tubes 9A, 9B, 9C and 9D is uneven, since the flow rate of the secondary air flowing through the air-injection tube arranged upstream of the air manifold 10 is greater than that through the air-injection tube arranged downstream of the air-manifold 10. The secondary air distribution ratio is gradually decreased from the air-injection tube 9D through the air-injection tube 9C and the air-injection tube 9B to the air-injection tube 9A in that order. Therefore, it is difficult to provide a uniform recombustion in the respective ports 3 and the branch pipes 6A, 6B, 6C and 6D. Owing to the unequal recombustion, the prior art exhaust-gas recombustion system cannot assure a sufficient oxidation treatment of the exhaust gases by means of the secondary air injection. The previously described Japanese Utility Model First Publication No. 59-141118 discloses a secondary air supply system optimally applicable for four-cylinder in-line engines in which one end of a single secondary air injection tube is connected to the exhaust manifold at the confluent point of the plural branch pipes. The Japanese document teaches the recombustion of the exhaust gases at the downstream ends of the respective branch pipes of the exhaust manifold. The above conventional secondary air supply system also cannot provide a sufficient recombustion of the exhaust gases because the exhaust-gas temperature at the downstream end of the branch pipe is lower than that at the upstream end of the branch pipe and in addition the secondary air is merely introduced into the confluent point of the downstream ends of the branch pipes. In addition to the above, the previously noted conventional exhaust-gas recombustion system with a manifold type catalytic converter teaches a separate arrangement of branch pipes of an exhaust manifold. Since the adjacent branch pipes are arranged independently of each other, the total surface area of the respective branch pipes 6A, 6B, 6C, and 6D is relatively great. This promotes heat radiation from each of the branch pipes 6. Consequently, the exhaust-gas temperature tends to be easily lowered, due to the above independent arrangement of the branch pipes. Thus, the temperature-rise of the catalyst support in the manifold type catalytic converter is delayed during a cold start of the engine. As a result, a catalytic conversion efficiency is necessarily lowered.